For third generation infrared detector technology, todays rival infrared photon detectors are HgCdTe (Mercury Cadmium Telluride or MCT) photodiodes and Quantum Well Infrared Photodetectors (QWIPs). However, QWIPs have high thermal generation rates causing higher dark currents. Increasing the operating temperatures above 77K, and thermionic emission dominates the dark current which drastically increases noise. The signal to noise ratio is therefore decreased. QWIPs are required to be cooled down to cryogenic temperatures (77K and lower) to be operated. The other types of detectors used in the infrared (IR) range are HgCdTe (Mercury Cadmium Telluride or MCT). MCT has inherent problems of thermal instability and poor compositional uniformity over large areas and it is one of the most difficult materials to be grown and processed due to the low strength of the material and surface uniformity making this option a high cost technology. Micro fabrication QWIPs are difficult since they can not detect direct incidence and grating is required. They have disadvantages due to cooling problems, bulk, volume, cost, and power consumptions. On the other hand, it is difficult to have uniform surface over large areas since MCT has thermal instability, poor compositional uniformity and poor interfaces. These problems originate from weak bonding characteristics of II-IV semiconductors and from Hg vapor pressure. Weak bonding reduces the strength of material, resulting in bad mechanical properties and creates difficulty in processing. Moreover composition control and mechanical problems result in difficulties in material processing and produces focal plane array (FPA).
On the other hand, most promising III-V material group is based on the versatile 6.1 Å (unit cell length) materials family have constituents of GaSb, InAs, AlSb which enables closely lattice matched bulk and superlattice (SL) layers. In type-II SL based infrared detector designs, most commonly used material is InAs/GaSb in a pin (p-type-intrinsic-n-type) detector structure since the type-II material system enables special distributions of InAs and GaSb layers. In such system, conduction band of the InAs stays below the valence band of the GaSb.
The effective band gap of the InAs/GaSb SL is defined as the energy separation between the lowest conduction and the highest valence minibands. The absorption wavelength can be adjusted approximately between 3 to 30 μm by altering layer thickness to cover wavelength range from mid to very long infrared. The type-II inter-band absorption is performed by electrons from conduction band and holes from valence band in the InAs/GaSb interfaces.
It is well known that there are InAs/GaSb (Indium Arsenide/Gallium Antimonide) superlattice pin structures used to detect rays of infrared wavelength and these are used in applications like thermal imaging systems etc. but the conventional InAs/GaSb superlattice pin structures have high surface leakage currents, therefore they require sufficient passivation. Beside these, InAs/GaSb interfaces have some bonding problems causing GR (generation/recombination) and diffusion currents resulting in an increase of dark current density. In the previous InAs/GaSb type-II pin SLs, detectivity decreases since the significant overlap of electron and hole wave functions can not be realized properly and the detectivities reside in the range 2×1010-6×1011 cmHz1/2/W at 77K.
The current methods with InAs/GaSb type-II pin SLs do not offer better carriers confinement, have high dark current densities, low dynamic resistances, low detectivities, require very low operating temperatures, Additionally, known methods have higher costs, higher operating voltages and require higher power consumption for cooling.
The Chinese patent document CN101562210, an application in the state of the art, discloses a GaAs-based InAs/GaSb superlattice infrared photodetector with a wave band of 3 microns to 5 microns and a manufacturing method thereof wherein GaSb buffer layers are grown on the GaAs substrate, and InAs/GaSb superlattices are grown on the GaSb buffer layers.
The United States patent document US2002125472, an application in the state of the art, discloses a multispectral radiation detector for detecting radiation in at least two spectral bands, comprises at least first and second photodiodes, each photodiode having at least one strain-compensating superlattice absorbing layer substantially lattice matched to adjacent layers of the detector.
The United States patent document US2004142504, an application in the state of the art, discloses a type-II superlattice photon detector, focal planes array and method for making. It is either a binary or tertiary system with a type-II band alignment comprising a system selected from the groups consisting of InAs/GaSb; SiGe; InAs/GaxIn1−xSb; and InAs/GaSb/AlSb.